116 J. Thos. Patterson. 



Experiment XII. 

 This experiment is similar to the preceding, except that the opera- 

 tion was made 30 degrees to the right instead of twenty (Fig. XXI,e), 

 and in the resulting embryo the injury is found in the right side of 

 the posterior end of the primitive streak (Fig. 69). 



Experiment XIII. 



An injury made still farther laterally (Fig. XXI,(i) does not affect 

 the embryo (Fig, 70), but is found in relatively the same position as 

 that in which it was performed. 



The results obtained from this set of experiments can leave no 

 doubt concerning the presence of portions of the dorsal lip which 

 lie between the boundary of the two areas, and which have not yet 

 completely fused together at the time when the egg is laid. The 

 manner by which this region is established has been considered in 

 connection with an earlier stage (Fig. XV). It is this structure 

 doubtless that Koller ('79) has described for the unincubated chick 

 blastoderm, and which is often called Roller's crescent. In the pig- 

 eon I have never observed a "erescentric groove" as described by 

 Koller,^° and furthermore this region does not give rise to the entire 

 primitive streak, but only to the posterior part. Even then it 

 usually completely disappears three or four hours before the primi- 

 tive streak becomes visible. 



V. Discussion and Summary. 

 Discussion. 

 Throughout the foregoing pages the term gastrulation is employed 

 to designate the process of invagination by which the gut-entoderm 

 takes its rise, together with the concomitant phenomenon of con- 

 crescence. It may therefore seem to be used in a sense that does not 



="1 refer here to the presence of a groove cluriiig the early hours of incu- 

 bation. It is true that in later stages (e. g., Fig. 20) one sometimes finds 

 the posterior end of the primitive strealv bifurcated, but these must be 

 regarded as much delayed cases, and are similar to those figured by Schanins- 

 land ('03) for the sparrow. 



